As safe as Transporters may seem, remember that terrorism still exists, and devices called "pattern scramblers" (DS9 Empok Nor.) are effective and deadly which can turn any transporter into a suicide booth without the transportee being at all aware that s/he is signing their own death warrant.

Like dominoes, state after state is going to have fucked up tap water eventually, but until that moment of transition, in every specific geographical location continentally vs. time, it's unclear why anyone would should stop drinking/using safe drinking water while it is still safe, before it is unsafe.